Control circuit for a lifting magnet

ABSTRACT

A control circuit for a lifting magnet which rapidly and automatically removes the residual magnetism in the magnet when the lifting current is released so that the magnet may be quickly unloaded. Provision is also provided for a fan control wherein the residual magnetism is not removed.

United States Patent 11 1 Strobl et al.

[ Jan. 7, 1975 [54] CONTROL CIRCUIT FOR A LIFTING 3,350,609 10/1967 Steele 317 123 MAGNET [75] Inventors: Fred A. Strobl, Chicago; Louis F. Primary EXami'WF-l Miner Ranieri, Niles, both f m Assistant Examiner-Harry E. Moose, J r. Attorney, Agent, or Firm-Hill, Gross, Simpson, Van [73] Asslgnee: Kory lndustrles, Inc., Ch1cago, Ill. Same", Steadman Chiara & Simpson [22] Filed: Nov. 27, 1973 [21] Appl. No.: 419,398 [57] I ABSTRACT A control circuit for a lifting magnet which rapidly US. Cl. and automatically rgmoves the residual magnetism in Cl. the magnet when the current is released so Fleld of Search the magnet may be unl0aded Provision is also provided for a fan control wherein the residual magne- Refel'ences Clted is not removed UNITED STATES PATENTS 3,043,990 7/1962 Lillquist 317/123 3 l Draw'ng F'gure CONTROL CIRCUIT FOR A LIFTING MAGNET BACKGROUND or THE nvENrioNi 1. Field of the Invention g r I This invention relates'in' general to controllers for,

. electromagnets and in particular to an automatic circuit for rapidly removing the residual magnetism ofa that the magnet cannot be utilized as rapidly as possible.

SUMMARY OF THE INVENTlON The present invention provides an automatic control circuit for an electromagnet wherein the-lifting current is applied in response to closing of a lift switch, and the load is dropped by depressing a drop switch which automatically and rapidly provides for demagnetizing the magnet which partially dissipates the magnetic residual voltage in a parallel resistor which further includes a relay which is actuated, by the reverse voltage thus causing contacts to close to apply reverse polarity current to the magnet. A fan circuit is also provided which prevents the application of reverse polarity voltage to the magnet when a load is dropped.

i 22 and the diodes D2, D4,and B6. A resistor R2 is connected from. fast: is 'to fuse 30-. A diode D8 is con- Other objects, feautures and advantages of the invention will be dily apparent from the followinggde taken in conjunction with the accompanying drawing,

scription of certain preferred embodiments thereof,

concepts of the-disclosure, and in which:

BRIEF DESCRIPTION OF THE DRAWING nected between leads 21 and 22 and poled as shown. Diode D8 is a selenium rectifier type with zener characteristics as for example the type sold under the trade marks Transorb, SURGE SUPPRESSOR or Volt Trap. Lead 21 is the positive supply bus and lead 22 is the negative supply bus. I I p In the following description, the switch contacts controlled by the various relay will be initially described and are shown in the FIGURE before the magnet has been energized. Normally opened switch contacts 31a is connected in series with a resistor R3 and normally opened relay contacts 33b between leads 21 and 22.

Normally opened contacts 33a are connected in series with resistor R7 and normally opened switch contacts 31b between leads 21 and 22. An indicator light 36 is connected between leads 21 and 22 and indicates when power is supplied to the leads 21 and 22. Normally closed contacts 32b are connected in series with normally opened contacts 34a and relay 33 and normally closed contacts 31c between leads 21 and 22. Normally opened switch contacts 32a are connected in series with normally closed drop switch 24 and' normally closed fan switch 26 which is in series with relay 31 which is in series with normally closed switch contacts 33c between leads 21 and 22. A lift indicator light 39 is connected across the relay 31 and switch contacts 33c. Relay 32 is connected between the junction between relay 31 and switch contacts 33c and the junction point between switches 24 and 26.

The electromagnet 38 has its winding connected between the junction points of contacts 31a and resistor R3 and the junctionpoint between resistor R7 and contacts 3lb.-A resistor R4 and a selenium rectifier with zener characteristics are connected in parallel although variations and modifications maybe effected without departing from the spirit andse'ope ofthe'hovel The FIGURE is'a schematic view of the inventions DESCRIPTlON or THE PREEERRE'D EMBODIMENTS phase AC power source and supply power through the switch contacts 51, 52, and 53 to the primary 10'of'the delta connected windings 11, 12, and 13. Secondary 14 has connected windings 16, 17, and 18 and is con-' nected to a rectifier 20 so as to provideDC voltage between leads 21 and 22. The rectifier 20 includes diodes D1, D2, D3, D4, D5, and D6 with diodes D1 and D2 connected in series and with a resistor R1 and capacitor C4 connected in series and the combination in parallel with the diodes D1 and D2. The end of winding 18 is connected to the junction point between diodesDl with the winding of magnet 38. A relay 34 is connected from the junction point between resistor R4 and diode D9 and a pair of variable resistors R5 and R6 which are connected in series with their opposite side connected .to a junction between contacts 31b and resistor R7.

, 1 34a. Terminals L1, L2, and L3 are connected to a three 1 'ln operation, switch 23 is closed to energize relays 31 and 32. Relay 31 closes contacts 31a and 31b which applies positive power through contact 31a to the magnet and D2. Diodes D3 and D4 are connected in seriesand nected to lead 21. A fuse 30 is connected between lead 38 and the negative side of the power supplyis connected to the magnet through contact 31b. The relay 31 also opens contact 310. At the same time, relay 32 closes contact 320 which is in parallel with the lift switch 23 so as to hold power to the relays 31 and 32. Relay 32 also opens contact 32b which prevents relay 33 from" being energized when relay 32 is energized.

Power'is applied to the magnet 38 which is then oper-, ated' with full applied power so as to lift material in a normal manner.

-When the load is to be dropped, switch 24 is opened by its push button whichdeenergizes relays 31 and 32.

When relay 31 is deenergized, contacts 31a and 31b open to disconnect power from leads 21 and 22 to the magnet and contact 31c is closed. When relay 32 is deenergized, contact 320 is opened and contact 32!) is closed.

When the contacts 31a and 31b are open, the applied power is removed from the magnet 38 which causes the magn'etic voltage'to reverse and the voltage developed across the magnet 38 will dissipate through resistor R4 and diode D9. The voltage developed across the resistor R4 due to this current, energizes relay 34 which closes contact 34a 'to energize drop relay 33. Drop relay33 closes contacts 330 and 33b which-applies the DCvoltage from leads '21-and122 in the reversepolarity. Initially when relay33is. energized since there are about 600 volts'developed across the magnet, no current will flow from the power terminalssince the power terminal voltage may be about'230fvolts'until the potential across the .magnet decreases'below 230 volts. When the voltage across the magnet 38-drops to below 230 volts, current will flow through contacts33a, resistor R7, the magnet 38, resistor R3, and contacts 331;. It is to be noted that this current is in the direction opposite that used to initially energize the magnet.

Since the resistor R4 is in parallel with the magnet 38, the relay 34 will remain energized until the voltage across the magnet 38 has dropped'to a relatively low value and in a operating magnet the relay R4 has been set by adjustment of. the variable resistors R5 and R6 to drop out atabout 46 volts. At that time, relay 34 deenergizes which opens up contact 34a. This deenergizes relay'33 which opens up contacts 33a and 33b and closes contact 33c. At this time, the circuit has returned to its initial condition before-the lift switch 23 was closed and theresidual magnetism in the magnet 38 has been reduced substantially to zero. The resistors R4, R5, and R6 are adjusted so that with an applied voltage of 230 volts from the power supply, the voltage would be dropped to about 30 volts across relay v34. As soon as relay 33 is deenergized, the current from the magnet 38 will be dissipated through resistor R4 and diode D9. The characteristic of the diode D9 is the same as diode D8 and current goes in the reverse direction through the diode D9 and resistor R4, and the diode D9 limits the voltage across the magnet to a safe voltage of about 400 volts.

The fan circuit is controlled by switch 26 and is used when it is desired to remove the magnet from the power supply without reversing the currentto it. When this operation is desired, the magnet will have. been energized by closing switch 23 so as'to apply current to the magnet through contacts 3la'and 31b as described previously. When the fan'switch 26 is depressed to open switch 26, the lift relay 31 will, bedeenergized thus opening contacts 31a and 31b and closing contacts 310. This removes the power supply from the magnet 38 and it starts discharging through resistor R4 and diode D9 and the relay 34 is energized. Howevensince relay 32 is still energized and the contact 32b is open, the drop relay 33 will not be energized and reverse polarity'of the power supply'will not be applied to the magnet 38.

When the fan button switch 26 is released and closes, the relay 31 will immediately be energized and will apply energizing current to the magnet 38 through contacts 31a and 31b. i

In a practical embodiment, a control circuit of this invention has been constructed as a-fully contained integrated system and comprises a substantial improvementover. the prior art magnetic control systems.

The resistor R4 may be "a non linear resistor which increases its resistance as the-voltage applied to it decreases. e Although this system has been described with respect to preferred embodimentsit is not to be so limited so magnet, a second switch connected between the second terminal of said power supply and the other side of the electromagnet, afirst relaycontrolling said first and second switches, a third switch connected in circuit with said first relay and said power's'upplyto energize it, a first-resisto -a unilateral current device connected in series with said first resistor and the electromagnet in parallel with said first resistor and unilateral current device, a second relay having an energizing coil, a second resistor connected in series with energizing coil of said second relay and said first resistor in parallel with the energizing coil of said second relay'and said second resistor, a third relay, a fourth switch controlled by said second relay and connected to saidthird relay and the combination connected across said power supply, a fifth switch controlled by said third relay and connected between said one terminal of said power supply and said other side of said electromagnet, a sixth switch controlled by said'third relay and connected between said second terminal of said power supply and said one terminal of said electromagnet, a third resistor connected between said fifth switch and said other side of said electromagnet, said unilateral current device is a selenium rectifier with zener characteristics, and a fourth relay. withone terminal connected to one terminal of the first relay, and a seventh switch connected I between the other terminal of said fourth relay and the other terminal of said first relay, an eighth switch controlled by said fourth relay and connected in circuit with said third relay.v 1 a 2. A control circuit according to claim l including a ninth switch controlled by fourth relay and connected in parallelzwith said third switch, a tenth switch with I one terminaliconnected to the ninth switch and the other terminal connected to the seventh switch.

3. Acontrol circuit for an electromagnet comprising, a direct current power supply with first and second terminals, a first switch connected between the one terminal of saidpower. supply and one side-of-said electrom'agnet,.a second switch connected between the second terminal of said power supply and the other side of the electromagnet, a first relay controlling said first and second switches, a third switch connected in circuit with said first relay and said power supply to energize it, a first resistor, a unilateral current device connected in serieswith said first resistor andthe electromagnet in parallelwith said first resistor and unilateral current device, a second relay having an energizing coil, a second resistor connected in series with energizing coil of said second relay and said first resistor in parallel with the'energizing coil of said second relay and said second resistor, a-third relay, a fourth switch controlled by said second relay and connected to said third relay and the combination connected across said power supply, a fifth switch controlled by said third relay and connected-between said one terminal of said power supply andsaid other side of said electromagnet, a sixth switch controlled by said third relay and connected between said second terminal of said power supply and said one is variable so as to set the deenergization point of said second relay, and said first resistor is non linear such that it increases its resistivity as the applied voltage decreases. 

1. A control circuit for an electromagnet comprising, a direct current power supply with first and second terminals, a first switch connected between the one terminal of said power supply and one side of said electromagnet, a second switch connected between the second terminal of said power supply and the other side of the electromagnet, a first relay controlling said first and second switches, a third switch connected in circuit with said first relay and said power supply to energize it, a first resistor, a unilateral current device connected in series with said first resistor and the electromagnet in parallel with said first resistor and unilateral current device, a second relay having an energizing coil, a second resistor connected in series with energizing coil of said second relay and said first resistor in parallel with the energizing coil of said second relay and said second resistor, a third relay, a fourth switch controlled by said second relay and connected to said third relay and the combination connected across said power supply, a fifth switch controlled by said third relay and connected between said one terminal of said power supply and said other side of said electromagnet, a sixth switch controlled by said third relay and connected between said second terminal of said power supply and said one terminal of said electromagnet, a third resistor connected between said fifth switch and said other side of said electromagnet, said unilateral current device is a selenium rectifier with zener characteristics, and a fourth relay with one terminal connected to one terminal of the first relay, and a seventh switch connected between the other terminal of said fourth relay and the other terminal of said first relay, an eighth switch controlled by said fourth relay and connected in circuit with said third relay.
 2. A control circuit according to claim 1 including a ninth switch controlled by fourth relay and connected in parallel with said third switch, a tenth switch with one terminal connected to the ninth switch and the other terminal connected to the seventh switch.
 3. A control circuit for an electromagnet comprising, a direct current power supply with first and second terminals, a first switch connected between the one terminal of said power supply and one side of said electromagnet, a second switch connected between the second terminal of said power supply and the other side of the electromagnet, a first relay controlling said first and second switches, a third switch connected in circuit with said first relay and said power supply to energize it, a first resistor, a unilateral current device connected in series with said first resistor and the electromagnet in parallel with said first resistor and unilateral current device, a second relay having an energizing coil, a second resistor connected in series with energizing coil of said second relay and said first resistor in parallel with the energizing coil of said second relay and said second resistor, a third relay, a fourth switch controlled by said second relay and connected to said third relay and the combination connected across said power supply, a fifth switch controlled by said third relay and connected between said one terminal of said power supply and said other side of said electromagnet, a sixth switch controlled by said third relay and connected between said second terminal of said power supply and said one terminal of said electromagnet, a third resistor connected between said fifth switch and said other side of said electromagnet, said unilateral current device is a selenium rectifier with zener characteristics, a fourth resistor connected between said sixth switch and said one terminal of said power supply, said second resistor is variable so as to set the deenergization point of said second relay, and said first resistor is non linear such that it increases its resistivity as the applied voltage decreases. 